Re–Os depositional age for Archean carbonaceous slates from the southwestern Superior Province:...

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Re–Os depositional age for Archean carbonaceous slates from the southwesternSuperior Province: Challenges and insights

G. Yang a,⁎, J.L. Hannah a,b, A. Zimmerman a, H.J. Stein a,b, A. Bekker c,d

a AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USAb Geological Survey of Norway, 7491 Trondheim, Norwayc Geophysical Laboratory, Carnegie Institute of Washington, Washington, DC 20015, USAd Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

a b s t r a c ta r t i c l e i n f o

Article history:Received 3 July 2008Received in revised form 9 January 2009Accepted 12 January 2009Available online 14 February 2009

Editor: R.W. Carlson

Keywords:Re–OsshaleSuperior Provincegeochronology

Rhenium–osmium (Re–Os) dating of carbonaceous rocks using H2SO4-CrO3 dissolution provides a powerfulnew geochronologic tool for Archean terranes. Here we document the oldest Re–Os depositional ages yetreported for carbonaceous sedimentary rocks. Carbonaceous slates are intercalated with graywackes andmafic tuffs of the Joy Lake sequence in the western Wawa subprovince of the Superior Province. Well-developed D2 cleavage is sub-parallel to bedding. Pyrite occurs in nodules, disseminations, and bedding-parallel stringers. Samples from two drill holes (DH26503 and DH26506) yield isochron ages of 2695±14 Maand 2684±16 Ma and initial 187Os/188Os ratios (Osi) of 0.15±0.16 and −0.29±0.20, respectively. Theages, identical within uncertainty, provide the first temporal constraints for the westernmost Wawasubprovince greenstone sequence. The ages are consistent with cessation of mafic/ultramafic volcanismbefore ca. 2695 Ma in the western Wawa subprovince and D2 deformation before 2685 Ma farther east. TheJoy Lake sequence is likely the temporal equivalent of turbiditic graywackes deposited further east, bothsourced from arc volcanoes during collision of the Wawa and Wabigoon arcs along the southern margin ofthe Superior Province.A chondritic Osi (0.109 at 2.69 Ga) is expected for Archean shales, but the negative Osi for DH26506 requiresexplanation. Re, which is significantly more mobile than Os, may have been released from intercalated high-Re/Os mafic tuffs by local oxidation during recent or Cretaceous exposure to meteoric waters. The adjacentcarbonaceous rocks are a ready sink for mobile Re. A small Re addition can shift isochron data points tohigher 187Re/188Os ratios with minimal slope change. This increases scatter and uncertainty for the Osi, shiftsthe isochron to the right, and produces a spuriously low Osi intercept. If Re addition is recent, the age is notsignificantly affected. Notably, the slates in DH26506 which yield the negative Osi are intercalated withrelatively permeable and reactive lithic wackes and mafic tuffs, whereas the slates in DH26503 are in contactwith less permeable massive intrusive rocks. This process can be detected only if the true Osi isapproximately chondritic, and is more likely where reactive mafic rocks are prevalent—both the case forArchean greenstone belts.

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1. Introduction

Since early work by Ravizza and Turekian (1989), a number ofstudies have utilized Re–Os geochronology to determine the time ofdeposition for carbonaceous shales. Re and Os in carbonaceous shalesare mainly of hydrogenous origin because both Re and Os are fixed byreduction with organic matter at or below the sediment–waterinterface in anoxic environments (Ravizza et al., 1991; Ravizza andEsser, 1993; Cohen et al., 1999; Jaffe et al., 2002). The 187Re–187Osisotopic systemwith its long half-life (λ187Re=1.666×10−11; Smoliar

et al., 1996; Shen et al., 1996) can provide highly precise depositionalages for carbonaceous sedimentary rocks. Early methods for digestionof shales, however, introduced significant uncertainty in the ages. Inparticular, the common method of digestion in Carius tubes usinginverse aqua regia, dissolves or leaches the entire rock, including non-hydrogenous detritus. Mixing of Re and Os from both hydrogenousand non-hydrogenous sources reduces the overall accuracy andprecision of the isochron age and initial 187Os/188Os ratio (Osi).Hannah et al. (2004) circumvented this problem by analyzingsyndepositional sulfides, establishing a highly precise age forPaleoproterozoic shales. In many cases, however, sulfides haveexchanged Re and Os with fluids or organic material duringrecrystallization, and therefore yield post-depositional or spuriousages (Hannah et al., 2006). Selby and Creaser (2003) recommended

Earth and Planetary Science Letters 280 (2009) 83–92

⁎ Corresponding author. Tel.: +1 970 491 3816; fax: +1 970 491 6307.E-mail address: [email protected] (G. Yang).

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H2SO4-CrO3 as the digestion reagent to releasemainly hydrogenous Reand Os from organic matter in carbonaceous shales. This new methodyields significantly less scatter around 187Re–187Os isochrons and,therefore, more accurate and precise age and Osi data (Kendall et al.,2004; Hannah et al., 2006).

Osi in the hydrogenous fraction of carbonaceous shale reflects theOs isotopic composition of seawater at the time of deposition (Ravizzaand Turekian, 1989, 1992; Cohen et al., 1999). Because Re and Oscompounds are remarkably insoluble when reduced, but mobilizedunder oxidizing conditions, radiogenic Os can be delivered to theoceans only by oxidative weathering and transport from thecontinents. In the anoxic Archean environment, therefore, seawaterOs was dominated by chondritic sources, such as mid-ocean ridgehydrothermal systems and dissolution or leaching of cosmic dust(Hannah et al., 2004). Thus, we can anticipate a chondritic Osi inArchean carbonaceous shales. Several recent studies of Archeanshales, however, have produced sub-chondritic Osi (Anbar et al.,2007; this study; AIRIE, unpublished data). Here we propose thatrecent oxidation of reactive materials with high Re/Os ratios, such asmafic tuffs, during exposure to meteoric groundwater can release Rewhich is then reduced and incorporated into adjacent carbonaceousshales. This shifts the isochron to higher 187Re/188Os ratios andproduces a spuriously low Osi intercept.

For this study, we focus on Neoarchean rocks of the Deer Lakegreenstone belt near Bigfork, Minnesota, in the western Wawasubprovince of the Superior Province (Fig. 1). This belt comprisesthe Thistledew Lake sequence of Fe-rich tholeiitic basalt flows, rareclastic rocks, and hypabyssal sills, the overlying Joy Lake sequence ofsedimentary strata with intercalated mafic and felsic tuff layers, andthe intrusive peridotite–pyroxenite–gabbro sills of the Deer LakeComplex (Jirsa, 1990; Severson and Jirsa, 2005; Fig. 2). There are noage constraints on the deposition of Neoarchean supracrustalsuccessions in northern Minnesota, although broadly correlativesuccessions to the east in the Wawa subprovince of Canada are welldated (Corfu and Stott, 1998). In the Shebandowan greenstone belt(Fig. 1), the oldest Greenwater assemblage includes ca. 2720 Maultramafic to felsic volcanic rocks, peridotitic, gabbroic, and anortho-

sitic intrusions, and volcanogenic massive sulfide deposits. Theunconformably or structurally overlying ca. 2695 Ma Kashabowieassemblage contains felsic volcanic and volcaniclastic rocks intrudedby felsic plutons. The youngest ca. 2692–2689 Ma unconformablyoverlying Timiskaming-type Shebandowan assemblage contains calc-alkaline to alkaline volcanic and coarse-grained sedimentary rocksdeposited in pull-apart basins. This assemblage was intruded bytonalites and syenites at ca. 2690 Ma. Subsequent deformation duringthe 2685–2680 Ma Shebandowanan phase of the Kenoran Orogeny,related to collision of the Wabigoon and Wawa volcanic arcs, isexpressed by the regional D2 event (Fralick et al., 2006; Percival et al.,2006; Fig. 1).

Poor exposure and lack of geochronologic data for supracrustalsequences of northern Minnesota hinder construction of a geologicframework for the western part of the Wawa subprovince. Thiscompromises correlation and metallogenetic comparison with othersedimentary successions in the Wawa subprovince and SuperiorProvince. The Deer Lake Complex was the target of Cu–Ni explorationduring the late 1960s and early 1970s (Ripley, 1983). Althoughexploration did not define economic ore deposits in the Deer LakeComplex, the drill cores provide access to intercalated supracrustalrocks of the Joy Lake sequence (Severson and Jirsa, 2005). In thisstudy, H2SO4–CrO3 digestion is used to acquire the depositional age ofthe Joy Lake sequence. Our Re-Os ages for the Joy Lake sequencepermit correlation with other supracrustal sequences in the SuperiorProvince, constrain timing of the dominant regional deformationevent, and provide a maximum emplacement age for the Deer LakeComplex. In addition, the data point to a possible cause for spuriouslylow Osi determined from isochrons for Archean carbonaceous shales.

2. Geologic setting

The Joy Lake sequence is confined to the Deer Lake greenstone belt,which resembles and is likely broadly correlative with the classicNeoarchean greenstone sequence in the Vermillion district to the east(Sims, 1972). Supracrustal volcanic and sedimentary rocks arebordered on the northwest by poorly studied undifferentiated granitic

Fig. 1. Subprovinces of the southern Superior Province (from Card, 1990 and Percival et al., 1994) with locations of the areas referred to in the text shown.

84 G. Yang et al. / Earth and Planetary Science Letters 280 (2009) 83–92


rocks and on the south by the syenite of the Coon Lake pluton. Earlydeformation of the supracrustal rocks produced nappe-like isoclinalfolds whose overturned limbs are currently exposed at the surface(Jirsa et al., 1992). The F1 folds, associated D1 cleavage, and axial planarveins were refolded during the dominant D2 deformation andaccompanying greenschist facies metamorphism. Subsequent defor-mation (D3) generated brittle structures and a NE-trending fabric insyntectonic plutonic rocks.

Severson and Jirsa (2005) divide the supracrustal rocks into twosequences. The lower Thistledew Lake sequence consists of high-Fetholeiitic metabasalts and marks the stratigraphic base in this area.Although strongly deformed in the study area, exposures of theThistledew Lake sequence to the east show well-developed flows withmassive bases, pillowed interiors, and fragmented tops. The overlyingJoy Lake sequence comprises sedimentary and volcanic units, some ofwhich may be contemporaneous with the Thistledew Lake sequenceand are mapped as its lateral equivalents. The sedimentary unit at thetop of the sequence includes interbedded slate and graywacke withlesser amounts of carbonaceous slate (the focus of this study), felsic tuff,mafic tuff, and chert. All units display a well-developed cleavageattributed to the D2 deformation. Sparse development of S1 cleavage inthese units suggests that D1 occurred at least partly prior to consolida-tion of sedimentary units, leading Jirsa et al. (1992) to propose anaccretionary wedge setting for deposition and deformation. Thegraywackes comprise turbiditic Bouma A units, suggesting a relativelyproximal environment. The adjacent volcanic unit consists of dacite,andesite, quartz latite tuff and breccia, with sparse hornblende andesitetuff and amygdaloidal flows, locally interbedded with volcaniclasticgraywacke and conglomerate (Severson and Jirsa 2005). These rocktypes suggest a depositional setting proximal to a volcanic arc, in partsubaerial, with local deeper-water sediment-starved basins accumulat-ing carbonaceous muds in an anoxic Archean environment.

The supracrustal rocks are intruded by the Deer Lake Complex, aseries of interfingered mafic and ultramafic sills (Berkley et al., 1978;Ripley, 1979; Ripley et al., 1982; Ripley, 1983; Jirsa, 1990; Severson andJirsa, 2005). The intrusive rocks include layered peridotite–pyrox-enite–gabbro sills, a gabbroic body with an unexposed basal section,and ultramafic lenses. As a result of greenschist facies metamorphism,olivine is extensively serpentinized, pyroxene is altered to amphibole,and plagioclase is saussuritized. The Deer Lake Complex as defined bySeverson and Jirsa (2005) also includes a unit of ultramafic and maficvolcanic rocks comprising high-Mg tholeiitic, komatiitic, and ultra-mafic flows, some with spinifex-textured bases, along with commonlenses of carbonaceous argillite. Olivine and pyroxene in these flowsare largely pseudomorphed by talc and actinolite. Both the supra-crustal rocks and the Deer Lake Complex are cut by the NW-trending Kenora-Kabetogama diabase dike swarm (equivalent to theFort Frances dikes in Canada), which yields a baddelyite U–Pb age of2.076 +5/−4 Ga (Southwick and Day, 1983; Buchan et al., 1996).

3. Sampling and analytical procedures

The sampled drill holes both angle about 45° to the northwestthrough an overturned section (Fig. 3). The intersected unitsconsistently show nearly vertical dips and tops down hole (NW), asindicated by graded bedding, spinifex textures, and pillowed flows.The carbonaceous slates, which comprise up to 26% by volume ofsupracrustal rocks in drill cores, range from 0.3 to 300 m in thicknessand contain up to 10% pyrite, occurring as nodules, concretions,disseminated crystals, and bedding-parallel seams (Ripley and Nicol,1981; Severson and Jirsa, 2005). Both drill holes intersected intervalsof carbonaceous slate in the Joy Lake sequence. Contrasting contactrelationships are exhibited by the two cores, however (Fig. 3). InDH26503, the sampled interval is bracketed by massive gabbroic sills

Fig. 2. Geologic map of the study area and the location of sampled drill holes (from Severson and Jirsa, 2005). Insert showswith an asterisk the position of the mapwithin Minnesota.

85G. Yang et al. / Earth and Planetary Science Letters 280 (2009) 83–92


of the Deer Lake Complex, including a shear zone at the lower contactand a chilled margin at the upper (down hole) contact. In DH26506,the sampled interval is not adjacent to intrusive rocks. The upper(down hole) contact is with volcaniclastic lithic wacke and siltstone ofthe Joy Lake sequence, which typically includes intercalated felsic andmafic tuffs. The lower (up hole) contact is with mafic volcanic rocks ofthe Deer Lake Complex.

Pyrite, confined to finely laminated carbon-rich layers, occurs asnodules, disseminated grains, and laminated seams within thecarbonaceous slate. Local folding of the pyrite seams is attributed toD2 deformation, whereas brittle fractures in pyrite crystals formedduring the late D3 deformation. Fibrous pyrite crystals form radialtextures innodules that range from1mmto3 cm indiameter.Wrappingof fine layers of the carbonaceous slate around the pyrite nodulesindicates an early, pre-compaction origin for the nodules, and pressureshadows record subsequent deformation. The pyrite textures and δ34Svalues support an origin by bacterial sulfate reduction during earlydiagenesis (Ripley and Nicol, 1981) rather than by high-T hydrothermalprocesses related to the intrusive Deer Lake Complex. Several genera-tions of hairline carbonate veinlets are apparent, with early veins foldedby D2, and later, undeformed veins filling fractures.

Sampled drill core intervals were selected away from intrusivecontacts where potential thermal or mechanical disturbances wereminimal. Samples were selected over short drill core intervals (b2 m)to minimize potential temporal variations in the Osi ratio (Hannahet al., 2004). For DH26503, about 500 mg of powder was produced bymicro-drilling, using a new diamond drill bit for each sample.We havetested several lots of drill bits by wearing them down on pure quartzand analyzing the resulting grey powder; near blank levels of Re andOs were found. Nevertheless, care is taken that diamonds are notbadly worn during drilling, and that the metal does not contact therock. For DH26506, clean chips weighing less than 1 g were manually

pulverized with a corundum mortar and pestle. Replicate analyseswere made on adjacent chips, pulverized and dissolved separately.Inclusion of sulfide and calcite veins was minimized by micro-drillingunder a microscope or by selection of carefully inspected chips.

For dissolution, a CrO3–H2SO4 solution was made by dissolving0.2 g CrO3 per ml of 4N H2SO4. About 500 mg of the whole rockpowder were accurately weighed and loaded into Carius tubes with185Re and 190Os spikes and 15 ml of the CrO3–H2SO4 solution, thendigested at 240 °C for 36 h, following the procedure outlined in Selbyand Creaser (2003). Osmium was separated and purified by CHCl3solvent extraction and micro-distillation. After Os extraction, SO2

gas was bubbled through the remaining Re-bearing solution for3 min to reduce Cr6+ completely to Cr3+. Rhenium was then sepa-rated by anion exchange chromatography with 1 g Eichrom 1×8100–200 mesh Cl-form resin. Rheniumwas purified further by singlebead anion extraction.

Purified Re and Os were loaded onto Pt filaments and isotope ratioswere determined usingNegative Thermal IonizationMass Spectrometry(NTIMS) in peak-hopping mode on the NBS mass-spectrometer at theAIRIE laboratory, Colorado State University. Measured isotopic ratios ofRe and Os were corrected for isobaric oxygen interferences, massfractionation of Os (192Os/188Os=3.08271, Brandon et al., 1999), andblank contributions. Re–Os isochrons were constructed with Isoplot 3.0(Ludwig, 2003), using a 187Re decay constant of 1.666×10−11 year−1

(Smoliar et al., 1996) and calculated 2σ uncertainties.

4. Results

Samples from drill core DH26503 yield a 5-point isochron with aModel 1 age of 2695±14 Ma, Osi of 0.15±0.16, and MSWD of 0.80(Table 1, Fig. 4A). Re and Os concentrations range from 4.7 to 12.8 ppband 0.166 to 0.532 ppb, respectively. Samples from drill core DH26506yield an 8-point errorchronwith a Model 3 age of 2684±16Ma, Osi of−0.29±0.20, and MSWD of 3.8 (Table 1, Fig. 4B). Re and Os concen-trations range from 12.6 to 24.9 ppb and from 0.423 to 0.836 ppb,respectively. The isochron age for samples from drill core DH26503 isconsistent with that for samples from drill core DH26506 within theuncertainties. If the results from the two drill holes are combined on asingle 13-point errorchron, the result is aModel 3 age of 2697±40Ma,Osi of −0.23±0.55, and MSWD of 49 (Fig. 4C). The age is consistentwith the broad regional age constraints on this sequence. The higherrors andMSWD, however, clearly indicate real geologic scatter in the

Table 1Re–Os concentrations and isotopic compositions for Joy Lake carbonaceous slate.

AIRIErun #

Depth in Core Re, ppb Os, ppb 187Re/188Os 187Os/188Os Rho(m)

Drill core 26503ORG-42 304.8–305.4 5.439 (4) 0.1967 (4) 659 (2) 30.38 (12) 0.60ORG-43 304.8–305.4 4.738 (4) 0.1656 (3) 800 (2) 36.97 (14) 0.59ORG-44 304.8–305.4 6.024 (4) 0.2315 (3) 505 (1) 23.33 (8) 0.57ORG-45 306.3 15.12 (1) 0.5315 (10) 770 (2) 35.52 (13) 0.58ORG-46 306.9 12.80 (1) 0.4150 (9) 1343 (4) 61.74 (31) 0.61

Drill core 26506ORG-30 91.3 16.14 (1) 0.6697 (13) 356.1 (8) 15.98 (5) 0.64ORG-36 Replicate 16.14 (1) 0.6392 (16) 424 (1) 19.16 (8) 0.65ORG-31 91.4 12.60 (1) 0.4227 (7) 970 (2) 44.22 (17) 0.64ORG-37 Replicate 12.66 (1) 0.4249 (9) 946 (2) 42.98 (16) 0.65ORG-33 92.8 13.52 (1) 0.5118 (7) 496.4 (9) 22.37 (6) 0.61ORG-39 Replicate 13.46 (1) 0.5119 (13) 488 (1) 22.00 (10) 0.64ORG-34 92.8 24.94 (1) 0.8274 (10) 1012 (2) 45.88 (13) 0.61ORG-40 Replicate 24.93 (1) 0.8365 (14) 943 (2) 42.83 (15) 0.62

Numbers in parentheses are 2σ absolute errors for the last digit reported. Rho is thecalculated error correlation. All data are blank corrected. The total procedural blanks for Reand Os of the set of DH26503 are 17.29 pg and 0.307 pg with average 187Os/188Os ratio of~0.160, and those for the set of DH26506 are 97.1 pg and0.573 pgwith average 187Os/188Osratio of ~0.174.

Fig. 3. Generalized logs for the two drill holes, 26503 and 26506. Both drill holes angleto the northwest, and all indicators of stratigraphic tops are to the northwest. Thestratigraphy is therefore inverted in the logs. Gabbros, pyroxenites, and peridotitesintrude the sedimentary section, with chilled margins described at four contacts in DH26503. The greenstone contains mafic and ultramafic volcanic rocks (see text fordescription). Details are available in Severson and Jirsa (2005) and accompanyingmaterials held by the Minnesota Geological Survey.



data. Inspection of the insert in Fig. 4C shows that weighted residualsfor samples from DH26503 are consistently positive and the dataplot above the errorchron, whereas weighted residuals for samplesfrom DH26506 are negative and the data plot below the errorchron.The impossible, negative Osi ratio in Fig. 4B likely documents open-system behavior that has shifted the data points to the right (higher187Re/188Os and/or lower 187Os/188Os) with minimal impact on theisochron age, as discussed below.

5. Discussion

5.1. Precision and accuracy of the ages

The statistics on the isochron for DH26503 are excellent, and theage therefore likely reflects the depositional age. In our experienceand in published Re–Os results for shales, post-depositional dis-turbance increases both the isochron uncertainties and the MSWD.While the isochron age may reflect isotopic closure after compactionand perhaps early diagenesis, it is highly unlikely that these processesextended for more than the 14 m.y. uncertainty on the age. There areno reported instances of a fully reset isochron (e.g., by metamorphismor hydrothermal alteration) without significant scatter of the data.Moreover, as discussed below, the age of ca. 2695 Ma is consistentwith the existing regional geochronology.

Although the isochron for DH26506 also has an uncertainty of only0.6% for the age, theMSWD is slightly elevated (3.8), suggestingminordisturbance. The analytical uncertainties for these data reflect the bestmeasurement capabilities of the NBS mass-spectrometer used in thestudy. Instrumental uncertainties for isotopic ratio measurements areless than 0.04% for 185Re/187Re, 0.24% for 187Os/188Os, and 0.12% for190Os/192Os, although fully propagated analytical uncertainties,including error magnification, are up to an order of magnitudegreater. The calculated MSWD decreases as analytical uncertaintiesincrease, and slightly higher analytical uncertainties for DH26506would result in a Model 1 age with MSWDb1. Furthermore, if thedata for DH26506 are forced into a Model 1 regression, none of theparameters—age, initial ratio, uncertainties, or MSWD—changesignificantly. Thus, while we base our conclusions on the more reliableisochron age for DH26503, the two ages are statistically indistinguish-able, and there is no reason to doubt the validity of either one.

Quality of results may be dependent upon the amount of materialhomogenized prior to sub-sampling for dissolution (e.g., Kendall et al., inpress). Small-scale decoupling of parent and daughter isotopes bytransport or diffusion after deposition can introduce heterogeneities inOsi, thereby causing scatter on the isochron diagram. To overcome this,samplesmust be large enough to capture the potential isotopicmigration.At the same time, if too much material is homogenized, there may be novariation in 187Re/188Os, which also compromises quality of the isochron.Because our results were achieved by homogenizing less than 1 g ofmaterial, we suggest that for very old samples the errors introduced byparent and daughter decoupling are smaller than the uncertaintiescaused by other factors (e.g. ~0.31% uncertainty on the decay constant).

5.2. Secular variations in Re and Os concentrations

Re and Os concentrations in seawater reflect input from seafloorhydrothermal activity, meteoritic dust, and continental weathering.Because Re and Os are both redox-sensitive elements, their dissolutionand transport in surface environments depend on the redox state ofthe atmosphere. Under anoxic atmospheric conditions such asinferred for the Archean, seawater is expected to have low concentra-tions of both Re and Os, and Os sources dominated by chondriticcompositions (Yang et al., 2002; Hannah et al., 2004; Anbar et al.,2007). The low initial Osi for DH26503, although imprecise, isconsistent with a chondritic composition for seawater Os. Re and Osconcentrations and 187Re/188Os ratios are significantly higher thanthose of the average continental crust (Esser and Turekian, 1993; seeFig. 5) indicating larger Re and Os fluxes with higher 187Re/188Os ratiosthan those expected from simple recycling of Re and Os fromcontinental crust on the Archean anoxic Earth. The Re and Osconcentrations and 187Re/188Os ratios are also comparable to thosecited by Anbar et al. (2007) for the stratigraphic interval that providesevidence for “a whiff of atmospheric oxygen” prior to the earlyPaleoproterozoic Great Oxidation Event (Holland, 2002; Bekker et al.,

Fig. 4. Re–Os isochrons for the Joy Lake carbonaceous slates: (A) data for DH 26503,(B) data for DH 26506, and (C) all data combined. Symbols are larger than analyticaluncertainties. Insert shows the plot of weighted residuals. A positive value indicates apoint above the isochron; a negative value indicates a point below the isochron.



2004). Given the relatively high 187Re/188Os, however, there may bean alternative explanation for the enhanced Re–Os flux.

Fig. 5 illustrates secular variations of Re and 188Os concentrationsand 187Re/188Os ratios of published data on shales for samplesdissolved by the CrO3–H2SO4 method. 188Os, which is calculatedfrom published data, serves as a proxy for common Os. The illustrationshows that there is no consistent temporal pattern, and that the JoyLake Re and Os concentrations are among the lowest yet reported.Concentrations in individual samples are dependent upon a number offactors, including total organic carbon contents, sulfur concentrations,sedimentation rates, dilution by carbonate, uptake mechanism, and

analytical procedures. As none of these factors are fully constrained inthe comparisons made here, further pursuit of the argument is notwarranted. To demonstrate a short-term excursion in Re and Osconcentrations, a detailed stratigraphic profile of concentrations in asingle section is required, as was done for Re by Anbar et al. (2007).

While absolute concentrations are controlled by the above men-tioned factors, we might expect the 187Re/188Os ratios to be higher inArchean shales. This is because Archean seawater was dominated byhydrothermal sources with relatively high 187Re/188Os ratios. Fig. 5shows that indeed the two Archean data sets have the highest values,but range to relatively low values. We infer that Re and Osconcentrations as well as 187Re/188Os ratios in these two shale setsmay reflect enhanced hydrothermal circulation that accompanied ca.2.7 and 2.5 Ga mantle plume breakout events that are also responsiblefor a global peak in mafic–ultramafic magmatism, sea-level rise,deposition of organic matter-rich sulfidic shales, enhanced continentalweathering, and banded iron formations (Condie et al., 2001; Barleyet al., 2005). Furthermore, we suggest that fine-grained sedimentsdeposited during time intervals when surface environments were notinfluenced by mantle plume breakout events will have lower Re and Osconcentrations and 187Re/188Os ratios reflecting larger relative con-tributions of Re and Os from recycled continental crust. A much broadersampling of both Archean and younger shales is needed to explore thesehypotheses and to document a statistically significant difference.

5.3. Challenges in defining initial 187Os/188Os ratios

The Osi ratios given by isochrons for both DH26503 and DH26506drill cores have large uncertainties. In addition, within 2σ uncertainty,the Osi for DH26506 is impossibly low, below the chondritic value at2.69 Ga (0.1086). The high uncertainties could be caused by (1) inade-quate sampling or poor analytical uncertainties, (2) the long projectionto the intercept from high 187Os/188Os ratios of the data points, or(3) post-depositional disturbance. Small analytical uncertainties, goodprecision and apparent accuracy of the isochrons, and minimal changein results with exclusion of some data points all argue against the firstpossible cause.

Themost likely cause of the poor precision for the Osi in both cases isthe long projection from high measured 187Os/188Os ratios. A survey ofrecent literature illustrates this anticipated problem. Reported absoluteuncertainties for Osi for samples with measured 187Os/188Os less thanten are typically in the second decimal place (generallyb0.05; Hannahet al., 2004; Kendall et al., 2004; Selby and Creaser, 2005; Kendall et al.,2006), whereas those for samples with measured 187Os/188Os of 20 to100 or more are commonly in the first decimal place (generallyN0.1;Bierlein et al., 2006; Morelli et al., 2007; Anbar et al., 2007). Highmeasured 187Os/188Os may be the result of high 187Re/188Os (Morelliet al., 2007) and/or great age (this study; Anbar et al., 2007).

The Osi of DH26506 presents a more complex case, as it shows notonly a large uncertainty, but also a nominally negative 187Os/188Osratio. As noted above, the age for DH26506 is within analyticaluncertainty of that for DH26503, and the analytical uncertainties forindividual measurements are excellent. A Model 3 regression wasrequired for DH26506, however, and the MSWD is slightly elevated(3.8), suggesting that some geologic factor introduced scatter andcontributed to the low ratio. The low apparent Osi can be explained bya process that offsets the isochron toward the right (lower 187Os/188Osand/or higher 187Re/188Os). This requires either loss of 187Os, additionof 187Re, or both, long after deposition so that the Osi ratio is reducedbut the isochron age is minimally affected.

Loss of 187Os is an unlikely explanation. Re is mobilizedmore easilythan Os during oxidative weathering of carbonaceous sedimentaryrocks (Pierson-Wickmann et al., 2002; Jaffe et al., 2002; Dalai et al.,2002). In other words, the loss of Os should normally be accompaniedby even greater loss of Re. The result would be to shift the isochron tothe left instead of right, and to increase rather than decrease the Osi.

Fig. 5. Secular variations in Re and 188Os concentrations and 187Re/188Os ratios in shalesanalyzed by the CrO3–H2SO4 method, shown in a box-and-whisker plot. The center linemarks the median, the boxes show the second and third quartiles, and the whiskersshow the first and fourth quartiles. Two outliers are apparent. Data for the ExshawFormation from Selby and Creaser (2003) are the first published data using thisanalytical method, although there is no reason to doubt their accuracy. Data from AIRIE(unpublished data; see Hannah et al., 2008) are analyses of shungite, a uniquePaleoproterozoic “fossil oil” (migrated pyrobitumen) that contains more than 50%organic carbon. See text for details.



Furthermore, all of the drill core samples from DH26506 are fresh andclean, with no evidence of oxidation.

Relatively recent addition of 187Re offers an alternative explana-tion. The mafic and ultramafic rocks that occur in and adjacent to theJoy Lake sequence are a potential source (Fig. 1). These rocks are likelyto contain high concentrations of Re and Os, with mafic rockscontributing relatively high Re/Os ratios. Re is mildly incompatibleduring mantle melting, and is enriched in mantle-derived meltsrelative to highly compatible Os (Shirey and Walker, 1998). That is,crustal rocks typically have high Re/Os ratios relative to mantlematerial. For example, the 187Re/188Os ratio for fertile mantle is about0.4, whereas the same ratio in komatiites is greater than 1, and inMORB and OIB ranges from 20 to 5000 (Shirey and Walker, 1998).Consequently, the average presently eroding continental crust has a187Re/188Os ratio of ~50 (Esser and Turekian, 1993). Mafic rocks, suchas themafic tuffs and basalts found in the Joy Lake sequence, representmoderate degrees of partial melting, and therefore contain relativelyhigh concentrations of Re and Os, and also relatively high 187Re/188Osratios. Thus, significant concentrations of Re and Os are locallyavailable adjacent to the Joy Lake slates, and the potential contami-nantsmost likely have 187Re/188Os ratios comparable to or higher thanaverage continental crust.

To test our hypothesis, we can calculate the amount of Re that mustbe added to each sample to offset the Osi from chondritic to theisochron value of−0.29 (see derivations in Supplementary material).For this calculation, we assume that the isochron age remainsunchanged. For simplification, we assume that only Re is added tothe slate; the amount of 188Os remains unchanged. The amount ofadditional 187Re required for each point on the isochron is given by:

187Re� �

C=

187Os188Os

� �i Mð Þ

−187Os188Os

� �i Tð Þ

eλt2 − eλt1� � 188Os

� �M

where (187Re)C is the amount of 187Re added after deposition bycontaminating fluids, (188Os)M is the amount of 188Os measured in thesample, (187Os/188Os)i(M) is the Osi determined from the isochron,(187Os/188Os)i(T) is the true Osi, t1 is the time of deposition of the slate,and t2 is the time when Re was added to the slate. We assume that(187Os/188Os)i(T) is chondritic (0.1086 at 2695 Ma), t1=2695 Ma, andthat the contaminationwas recent (t2=0). The calculated amounts of187Re required to offset the Osi from chondritic to −0.29 range from0.07 to 0.25 ppb, or from 0.9 to 2.4% of the measured 187Reconcentrations. That is, the amount of Re contamination is small andthus has minimal impact on the precision of the isochron. Note,however, that the required change increases as t2 increases.

If the isochron age is unaffected by the contamination, the 187Readdition must be proportional to the 188Os concentration of eachsample. Small deviations from this proportionality, however, result inonly minor increases in the scatter, and thus in the uncertainties andMSWD. Indeed, the amount of added 187Re required to offset theisochron in the disturbed drill core is small, and the MSWD is slightlyhigher. Moreover, to avoid any change in the age, proportionally moreRe must be added to the samples with higher 188Os concentrations. Ifinsufficient Re is added to the samples with high 188Os concentrations,the result will be an apparent decrease in the age. Indeed, the age isnominally younger (thoughwithin uncertainty) for the disturbed drillcore. Of course, the assumption that no Os is added is almost certainlynot true. Because the contaminant source certainly had a much lower187Os/188Os than the shales at the time of contamination, however,addition of Os would complement rather than counter the addition ofRe. Our purpose is not to suggest a unique process or explanation forthe data. Rather, we show with relatively simple calculations thatminor contamination by a fluid with moderately high 187Re/188Osratios occurring relatively recently can significantly shift the apparentOsi in old samples with little impact on the isochron age.

Another obvious question arises. Why was DH26506 disturbedwhile DH26503was not? Fig. 3 shows that the samples fromDH26506were taken from a slate section at a shallow depth (about 70 m belowthe present surface in an inclined drill hole) bound by graywackes andvolcaniclastic rocks. These units include mafic tuffs, volcanic flows,and volcaniclastic rocks, which are relatively permeable and finegrained, and, therefore, reactive. As the section was exhumed anderoded to its present shallow depth, oxidizing groundwater couldreadily attack the more permeable rocks, releasing Re and lesseramounts of Os. The highly reducing environment of the adjacentcarbonaceous slate offers a ready sink for the mobilized, redox-sensitive elements (Siebert et al., 2005). In contrast, Fig. 3 shows thatthe samples fromDH26503, which are undisturbed, were taken from aslate section at a greater depth (about 220 m below the presentsurface in an inclined drill hole) bound by gabbro with a recognizedchilled margin at one contact. Not only do gabbros normally havelower concentrations of Re and Os than mafic tuffs, but they also havevery low permeability. It is therefore less likely that recentlyintroduced oxidizing groundwater could readily infiltrate throughthe gabbro at N200 m depth and impact the adjacent slate.

In the nearbyMesabi Iron Range, Minnesota, quartz- and kaolinite-rich profiles were generated by oxidation and intense chemicalweathering from the Late Jurassic through Early Cretaceous under awet, subtropical climate (Tyler and Bailey, 1961; Morey, 1983; 1999).This event may also have caused oxidation of mafic and ultramaficvolcaniclastic rocks in the Joy Lake sequence, releasing Re for uptakeby adjacent shales. The required amount of Re thatmust be introducedat 100 Ma is statistically indistinguishable from the amount requiredfor recent contamination (0.9 to 2.6% at 100Ma vs. 0.9 to 2.4% at 0Ma).For much earlier contamination, however, the required Re addition ismuch larger, and varies much more for each data point (e.g., 3.5 to9.4% at 2000 Ma). The more Re addition required, the greater theprobability that the age, Osi, and MSWD will be significantlydisturbed. Therefore, we favor a relatively recent contamination event.

Nominally lowOs initial ratios have been reported, both formally andinformally, for several Archean examples (this study; Anbar et al., 2007supplemental data; AIRIE unpublished data). Our calculations show thatthis is to be expected. Most analyzed shales of Neoproterozoic andyounger age have Osi significantly greater than chondritic values andranging from 0.13 to 1.1 (Kendall et al., 2004; Selby and Creaser, 2005;Kendall et al., 2006; Hannah et al., 2006, 2007; Creaser and Stasiuk,2007; Turgeon et al., 2007; Selby, 2007). Far more Re must be added tothese samples to offset the isochron to a sub-chondritic value similar tothat observed in this study (Fig. 6); it is therefore less likely. The

Fig. 6. Illustration of isochron shift resulting from increase in 187Re/188Os for each datapoint. To produce a parallel shift, with no change in the slope of the isochron, Remust beadded proportionately so that the change in 187Re/188Os is identical for each point. Therequired change in 187Re/188Os to generate a given Osi depends on the age at the time ofcontamination; a much smaller change (shorter arrows) is required for older samplesuites because of the steeper isochron slope.



increased scatter because of non-proportional addition of Re will be fargreater with large Re additions, so that the isochron is likely to bediscounted as disturbed. A small decrease in the measured Osi might beeasily overlooked; a small Re addition will simply shift the Osi to aslightly lower, but still reasonable value. Finally, Archean terranescommonly contain reactive, high-Re/Os mafic rocks, a ready source forcontamination that is less abundant in post-Archean sections.

5.4. Regional significance of geochronology

The 2695±14 Ma isochron age for samples from drill coreDH26503 is the oldest reported Re–Os age for carbonaceous slate orshale and the first depositional age for the Neoarchean supracrustalsuccessions in the western part of the Wawa subprovince in USA. Itprovides a robust age for sedimentation in the Deer Lake greenstone belt,and potentially places aminimum age constraint onmafic and ultramaficvolcanism and a maximum age on the Deer Lake intrusive complex.

Multiple models have been proposed for crustal thickening in theSuperior Province, including amalgamation of arc terranes (Langfordand Morin, 1976; Percival and Williams, 1989; Card, 1990; Thurstonand Chivers, 1990; Kimura et al., 1993; Kerrich and Polat, 2006) and aglobal mantle reorganization (Rey et al., 2003). Recent modelsincorporate both of these scenarios into a model of complex andlong-lived interaction between collisional processes in a subductionsetting and mantle plume breakout events (Ayer et al., 2002). Mostmodels for the Superior Province suggest that the D1 compressionalevent was roughly contemporaneous with deposition of arc volcanicand associated sedimentary rocks, and centered around 2695 Ma(Corfu and Stott, 1998). The ca. 2695 Ma Re–Os age for carbonaceousslates from the Joy Lake sequence links their deposition to this eventand permits correlationwith sequences of similar age to the east in theWawa and Quetico subprovinces (Fig. 1).

Throughout much of the Wawa and Quetico subprovinces, the ca.2695Ma sequences are dominated by volcaniclastic and felsic volcanicmaterials, sit unconformably or in fault contact with ca. 2720 Mavolcanic and sedimentary sequences, and are intruded by felsicplutons (Corfu and Grunsky,1987;Mueller et al., 1996; Corfu and Stott,1998; Zaleski et al., 1999; Davis et al., 2000). The sedimentary part ofthe Joy Lake sequence is probably a deeper-water (and/or locallystarved basin) time equivalent of the turbiditic graywackes of theQuetico subprovince and McKellar Harbour Formation, which overliesthe carbonaceous and pyritiferous Kingdom Slate in the northern partof the Wawa-Abitibi subprovince between Schreiber and Hemlo(Fralick et al., 2006). In contrast to the correlative successions furthereast in the Wawa and Quetico subprovinces (Schreiber-Hemlo area),the Joy Lake sequence is interlayered with mafic and ultramaficvolcanic rocks and intruded by mafic and ultramafic intrusions. Twoalternatives can be put forward to reconcile this difference inassociated magmatism and, likely, tectonic settings: (1) imbricationof terrigenous and oceanic materials of variable ages in an accre-tionary setting (Kimura et al., 1993), or (2) on-going distal arcvolcanism at ca. 2695 Ma outboard of, but simultaneous with north-ward-directed subduction under theWabigoon subprovince in a moreproximal part of the Superior Province.

In accretionary settings, imbrication of terrigenous sediments andsliced oceanic crust can generate allochtonous rather than auto-chthonous contacts between units of different ages representingdifferent tectonic settings. This has been already demonstrated in theMalartic and Larder areas in the southern Abitibi subprovince and inthe Beardmore-Geraldton area of the southeastern Wabigoon Sub-province (Kimura et al., 1993). Although similar relationships seemapplicable to the Deer Lake greenstone belt, we note that some maficand ultramafic intrusions have well-documented chilled zones,arguing against tectonic contacts. Furthermore, the intrusions lackpenetrative deformation (Severson and Jirsa, 2005), suggesting thatthey post-date the D2 event, and therefore, the Joy Lake sequence. It

bears noting, however, that some of these intrusions have a shearedcontact with the Joy Lake sequence (see Fig. 3). If some units areindeed allochtonous, then mafic and ultramafic volcanic units of theThistledew Lake and Joy Lake sequences could be older than the datedsedimentary rocks, and, perhaps, correlative with ca. 2720 Maultramafic to felsic magmatism in the Schreiber-Hemlo area farthereast in theWawa subprovince of Canada. This implies a ca. 25 Ma timegap between terrigenous and volcanic units in the Deer Lakegreenstone belt, similar to that documented in the Manitouwadgegreenstone belt to the east (Zaleski et al., 1999; Fig. 1).

The alternative and our preferred interpretation is that thesedimentary and magmatic rocks in the Deer Lake greenstone beltare autochthonous, with either depositional or intrusive contactslocally disturbed by faulting. If so, the ca. 2695 Ma age applies to boththe sedimentary and associated mafic to ultramafic volcanic rocks. Asimilar relationship was indeed documented in the Newton green-stone belt of the adjacent western part of the Vermillion district inMinnesota, where felsic and mafic volcanic rocks, volcaniclastic rocks,graywacke, iron formation, and impure marble are interlayered, andthe supracrustal sequence is intruded by diabasic gabbro and dif-ferentiated mafic to ultramafic sills (Schulz, 1980; Southwick et al.,1996). Lithologically similar 2700–2687 Ma sequences (Corfu et al.,1989; Mueller and Donaldson, 1992) in the southern Abitibisubprovince contain deep-water facies (turbiditic graywacke, carbo-naceous shales, and banded iron formations), are closely associatedwith ultramafic-mafic and felsic volcanic rocks, and are proximal tomajor east–west trending fault zones with gold mineralization. Thesesequences are confined to the southern (external) part the Abitibigreenstone belt, which is the eastern extension of the Wawasubprovince across the Kapuskasing uplift. Together, the Deer Lake,Vermillion, and southern Abitibi greenstone belts may reflectcontinuing arc volcanism in the distal part of the Wawa-Abitibisubprovince during its collision with the Wabigoon arc along thesouthern margin of the Superior Province (Fig. 1). This interpretation,if correct, would explain the apparent differences in metallogenybetween the Deer Lake greenstone belt and the ca. 2720 Ma succes-sions in the Wawa-Abitibi subprovince. Specifically, the supracrustalsuccession lacks volcanogenic massive sulfide deposits and theultramafic–mafic intrusions lack economic NiS mineralization, bothcommon in the older successions. Further geochronologic work andcareful documentation of contact relationships is needed to discrimi-nate between these two alternatives, as they have significant im-plications for the metallogenic potential of this belt.

The D2 event represents the collision between the Wawa andWabigoonvolcanic arcs during the Shebandowanphase of the KenoranOrogeny, which impacted the entire Superior Province (Card, 1990;Percival et al., 2006; Fig. 1). In Minnesota, to the east of our study area,dating of two phases of the Giant Range batholiths brackets the D2

event between 2685 and 2669 Ma (Boerboom and Zartman, 1993).Ages from the Shebandowan greenstone belt farther to the east of theDeer Lake Complex place the D2 transpressive event at 2685 to2680 Ma (Corfu and Stott, 1998), and stitching granitoids along theWawa-Quetico subprovince boundary in the Manitouwadge green-stone belt bracket the D2 event between 2687 and 2680 Ma (Zaleskiet al., 1999). The 2695 Ma age for deposition of the Joy Lake sequenceprovides a maximum age for the D2 event in the Deer Lake greenstonebelt, complements other age constraints for this event, and extends ageconstraints for the D2 event to the western limit of the Wawasubprovince. Because the intricately interlayered mafic and ultramaficsills of the Deer Lake Complex lack a penetrative fabric and showchilled, intrusive contacts with the deformed Joy Lake supracrustalrocks, they must have been emplaced after D2 deformation (Seversonand Jirsa, 2005), potentially at the same time as post-orogenic ca.2668 Ma composite ultramafic to felsic intrusions in the Gamitagamaand Batchawana greenstone belts on the east shore of Lake Superior(Fig. 1; Krogh and Turek, 1982; Corfu and Grunsky, 1987).



Mueller et al. (1996) propose two distinct evolutionary phases inthe Abitibi greenstone belt: (1) arc–arc collision between 2697 and2690 Ma, and (2) arc fragmentation between 2689 and 2680 Ma withthe formation of the successor strike–slip basin (Mueller et al., 1991).This model might also explain the evolution of the Deer Lakegreenstone belt. Mafic to felsic volcanism and associated volcaniclasticrocks of the Thistledew Lake and Joy Lake sequences were depositedcontemporaneously with D1 deformation in an accretionary setting.Carbonaceous muds accumulated in deeper-water environments insediment-starved basins. Transpressive amalgamation of arc terranesimposed D2 fabrics on the sequence. The mafic and ultramafic sills ofthe Deer Lake Complex intruded and interfingered with thesupracrustal rocks of the Joy Lake sequence during arc fragmentationin the aftermath of D2 event.

6. Conclusions

We report here the oldest Re–Os isochron age for carbonaceousslate or shale to date. The 2695±14 Ma Joy Lake carbonaceous slatewas deposited during or after D1, but before D2 deformation. Thevolcanosedimentary supracrustal succession of the Deer Lake green-stone belt may be correlative with similar successions to the east inthe Wawa subprovince and southern Abitibi belt. Together, thesesequences may represent on-going distal arc volcanism along thesouthern margin of the Superior Province during collision of the moreproximal part of the Wawa arc with the Wabigoon arc. The mafic andultramafic sills of the Deer Lake Complex post-date deposition of theJoy Lake carbonaceous slate. The negative Osi and apparently slightlyyounger isochron age of the DH26506 may result from addition of Rederived from mafic and ultramafic volcanic rocks during exhumationand oxidation.

Acknowledgments

We thankMark Severson, Precambrian Research Center, Universityof Minnesota, Duluth, for providing full details for the drill holessampled in this study. Funded by NSF awards EAR-0309197 and EAR-0451693 to HJS and JLH. AB's participation was supported by theNASA-Ames Research Center, NASA, and PRF/ACS grants to H.D.Holland and, at a later stage, by NSF grant EAR-05-45484, NASAAstrobiology Institute award No. NNA04CC09A, and a NSERC Dis-covery Grant to AB. The manuscript benefited significantly frominsightful reviews by Fernando Corfu and Robert Creaser and editorialhandling by Rick Carlson.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.epsl.2009.01.019.

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Re–Os depositional age for Archean carbonaceous slates from the southwestern Superior Province:...

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